1. Field of the Invention
The present invention relates to a mechanism for adjusting the transverse position of a member slidable along a shaft, the mechanism being particularly useful as a platen gap adjuster in a printer.
2. Description of the Prior Art
Electro-mechanical printers are widely employed as computer output devices or components of typewriter or word processing systems. One type of printer employs a print head which moves across the page to print each line one character at a time. Typically, the head can print any desired character at each print position. At the end of each line, the page is advanced and the print head again is moved across the page to print the next line.
Generally, impact printing is employed. To this end, the print head may employ a set of type fonts for each character, or a set of individual impact styli arranged in a matrix and selectively actuated to print any desired character. A typewriter-like ribbon, carbon paper or other inked media is interposed between the print head and the page. A platen is situated behind the page. Printing is accomplished as the head-mounted type font or styli impacts the ribbon against the page and platen.
Adjustability of the spacing or gap between the print head and the platen is desirable for several reasons. Such gap adjustment accommodates the use of different page thicknesses or number of copies. Thus, a greater spacing may be desirable when printing several "carbons" than when printing a single sheet. Further, the effective impact force can be controlled by adjusting the platen gap. A smaller gap will result in a stronger impact, which may be desirable for "darker" printing, or undesirable if e.g., it causes tearing or punching of holes through the ribbon. An object of the present invention is to provide an improved platen gap adjuster.
The difficulty in facilitating platen gap adjustment arises from the fact that the print head and its associated carriage must be free to move across the page. The platen gap adjustment direction is transverse to this movement, and must be made without interfering with that movement. One approach is to adjust the position of the print head with respect to the carriage itself. While such adjustment does not interfere with the movement of the carriage across the page, such adjustments are critical, have required the use of tools, and necessitate opening of the printer housing and the use of tools within such housing. Such arrangements are unsatisfactory, as they do not permit gap adjustment e.g., to compensate for different numbers of copies, by unskilled personnel.
A further object of the present invention is to provide a mechanism for accomplishing platen gap adjustment without the use of tools and without altering the mounted position of the print head on its carriage. The adjustment facilitates changing of the platen gap by as little as a few thousandths of an inch, repeatably and accurately, and without interfering with the movement of the carriage across the page. The adjustment can be made by an unskilled operator. The same adjustment also enables withdrawal of the print head away from the platen, and concomitant disengagement of the paper tensioning fingers, so as to facilitate paper insertion and removal.